An internal seal has been proposed in which a ring of L-shaped cross section is sealed against the recess in the face of the piston by means of an O-ring of elastomer material, seated under prestressed condition between the axial leg of the ring entering into the recess in the face of the piston and the radial inner wall of the recess.
It has now been found that an O-ring of that type, owing to insufficient long-term heat resistance of the elastomeric material does not serve its purpose over long periods of time. The O-ring loses its elasticity and eventually fails to seal.